"There are social and political challenges," said Joy Workman, a senior program manager with the National Institutes of Allergy and Infectious Diseases, at the annual U.S. Conference on AIDS in Anaheim, Calif.

In a series of video interviews presented at the conference, people in the major risk groups said they feared taking part in health research because they don't trust their government. Many summed up their concerns by mentioning the words "guinea pig" and "Tuskegee" -- a reference to the disastrous study of syphilis among Alabama black men.

The willingness of people to participate in vaccine testing is no minor matter, experts said. As many as 15,000 people must take part in the final testing phase of any HIV vaccine, said Steve Wakefield, associate director for community relations with the HIV Vaccine Trials Network. And it makes the most sense to launch the studies in developed countries that have strong health systems in case someone gets infected with HIV, he said.

Researchers have been testing HIV vaccines on people since 1987. Their goal is to develop a vaccine that will prevent infection by HIV, the virus that causes AIDS.

While researchers have studied 50 vaccines worldwide, only one has reached so-called "Phase III" testing, in which thousands of people at high risk are given the vaccine and then followed for several years to see if they become infected. The other phases, which last as long as two years each, test the vaccines on small numbers of people, usually at low risk, to determine if they are safe and if they work better than placebos.

Results from testing of the one vaccine in Phase III -- known as AIDSVAX -- are expected next year. It is being developed by the VaxGen company, which has been examining its effectiveness among subjects in North America, Puerto Rico, the Netherlands and Thailand.

There's reason to believe a vaccine is possible, because some people at extremely high risk -- prostitutes, for example -- don't get infected despite repeated exposure, said Dr. Gaston Djomand, a clinical trials physician with the HIV Vaccine Trials Network. Also, a small number of HIV-positive people have never developed AIDS despite being infected for many years.

"There is probably a natural mechanism of protection," Djomand said. "We strongly believe that a vaccine is possible."

However, researchers have had to abandon several vaccine strategies because they didn't work, he said. Researchers are no longer looking at using a live weakened strain of the virus (as is used in measles and chicken pox vaccines) or a dead inactivated virus (as in flu vaccines).

The most promising vaccines in development may be able to prime the immune system to protect against transmission of HIV through both sex and blood.

Vaccine experts said education is key to convincing people at high risk to take part in the final phases of HIV vaccine studies, which may become more common. "Just handing out a brochure will not bring out all the information you need," Wakefield said.

The challenges appear to be major, according to a video that studied opinions about vaccine studies among gay men in San Francisco, IV drug users in Philadephia and blacks in Durham, N.C.

"We're in a minority, and in the bigger picture, we're not treated that well by the government," said a gay man. "Are they really going to be honest with us?" asked a woman from Philadelphia.

And a male IV drug user asked, "What good is it going to do for you to cure me?"

Renee Turner, a television producer and member of a federal HIV vaccine communications task force, said researchers must find ways to get people at risk to stop worrying about "boogiemen" and think about the health of their community.

"There is a cost for being left out," she said.

What To Do

Learn more about HIV vaccine development from the HIV Vaccine Trials Network.

The Tuskegee study of black men with syphilis has cast a pall over government medical research. Learn more about its "troubling legacy" from the University of Virginia.